Fibromyalgia (FMS) is a common, poorly understood rheumatic disorder characterized by symptoms of chronic fatigue and musculoskeletal pain. Recent studies have suggested that muscle metabolism may be abnormal in FMS, although most of these studies to date have not adequately controlled the level of fitness. This study proposes to examine the hypothesis that abnormal muscle fatigue and metabolism are present in FMS, by utilizing a unique combination of two highly sensitive techniques: surface electromyography (EMG) and 31P-Nuclear Magnetic Resonance (NMR) spectroscopy. These two techniques will be used to simultaneously determine the precise electrophysiologic and metabolic characteristics of muscle in patients with FMS. The surface EMG data will provide an objective index that may have clinical applicability as an easily administered, non-invasive assessment procedure for muscle dysfunction associated with FMS. The proven capability and more direct methodology of NMR-spectroscopy will identify the phosphate components of muscle energetics and will validate the EMG methodology. Controls will be carefully matched for age, gender and level of activity. In addition, the level of aerobic fitness will be determined in both patients and control subjects. The electrophysiologic and high energy phosphate metabolite changes associated with muscle fatigue will be compared in patient and control populations at each muscle site. It is anticipated that these data will determine whether the metabolic and electrophysiologic properties of muscles in patients with FMS are abnormal.